Kilovoltage Intrafraction Monitoring

Tumours move during radiotherapy treatment. This results in reduced geometric and dosimetric accuracy. Typically, this motion is not observed during treatment. Real-time tumour localization and real-time adaptation may improve treatment outcomes.

Several real-time tumor localization modalities exist i.e. ultrasound, magnetic resonance imaging (MRI), optical, megavoltage (MV), combined MV/kV, Calypso electromagnetic guidance and the Navotek radioactive fiducial implant. However, some of these are under development, not easily available or expensive. An ideal solution would be to use existing linear accelerator (linac) systems. Hence, the need for a novel real-time tumor localization modality is twofold. First, it facilitates real-time beam adaptation modalities (e.g. real-time tumor tracking). Second, it can be used for real-time image guidance.

Kilovoltage intrafraction monitoring is a novel intrafraction real-time tumor localization modality. It involves a single gantry-mounted kV x-ray imager (widely available on most linacs) acquiring 2D projections of implanted fiducial markers. 3D positions are then reconstructed by maximum likelihood estimation of a 3D probability density function. This study aims to clinically implement this novel real-time tumor localization modality for the first time for patients with localized prostate cancer undergoing intensity modulated arc therapy (IMAT).

Method

The figure below shows the workflow of the clinical study. As the linac gantry rotates around the patient during treatment (1), the kV imager acquires 2D projections of the prostate (2). The fiducial markers are segmented using an in-house developed software package (3). 3D positions are determined via maximum likelihood estimation (MLE) of a 3D probability density function (pdf) (4). The 3D trajectory of the prostate is then plotted as a function of time (5).

Results

The sub-milimeter accuracy and high spatial and temporal resolution of Kilovoltage Intrafraction Monitoring allows for the determination of a wide variety of prostate trajectory types. These trajectories are displayed in the figure below.

The centroid (of the three fiducial markers) offset from the initial position is plotted as a function of time. The 3 Dimensional (3D, black), Superior-Inferior (SI, blue), Anterior-Posterior (AP, green) and Left-Right (LR, red) displacements are plotted. The trajectory is divided into three sections namely the pretreament, first and second arcs. The first and second treatment arcs are grayed out.

Conclusions and future work

Kilovoltage Intrafraction Monitoring is a novel real-time tumour localization modality which has successfully been implemented clinically on a patient cohort of 10 patients. Several advantages of Kilovoltage Intrafraction Monitoring include:

Being the most accurate real-time tumour localization modality in the world so far (0.46 mm)

Low barrier to clinical implementation

Widespread applicability

However, one disadvantage of Kilovoltage Intrafraction Monitoring is that it delivers kolovoltage dose to the patient.

Kilovoltage Intrafraction Monitoring allows for the position of the tumour to be determined in real-time. This information can be used to adapt to the motion of the tumour via several strategies. Some of these strategies include radiation beam gating and real-time tumour tracking.